Failed Switch Recovery - Cisco Catalyst 4500 Series Software Configuration Manual

Understanding Virtual Switching Systems
Figure 5-6

Failed Switch Recovery

If the VSS Active switch or supervisor engine fails, the VSS initiates a stateful switchover (SSO) and
the former VSS Standby supervisor engine assumes the VSS Active role. The failed switch performs
recovery action by reloading the supervisor engine.
In Quad-Supervisor VSS mode, if the VSS Active switch or supervisor engine fails, the VSS initiates a
stateful switchover (SSO) and the former VSS Standby supervisor engine assumes the VSS Active role.
The in-chassis standby (ICS) on the failed switch becomes the VSS Standby and the former VSS Active
becomes the ICS for the VSS Standby.
If the VSS Standby switch or supervisor engine fails, no switchover is required. The failed switch
performs recovery action by reloading the supervisor engine.
The VSL links are unavailable while the failed switch recovers. After the switch reloads, it becomes the
new VSS Standby switch and the VSS reinitializes the VSL links between the two switches.
The switching modules on the failed switch are unavailable during recovery, so the VSS operates only
with the MEC links that terminate on the VSS Active switch. The bandwidth of the VSS is reduced until
the failed switch has completed its recovery and become operational again. Any devices that are
connected only to the failed switch experience an outage.
Note The VSS may experience a brief data path disruption when the switching modules in the VSS Standby
switch become operational after the SSO.
After the SSO, much of the processing power of the VSS Active supervisor engine is consumed in
bringing up a large number of ports simultaneously in the VSS Standby switch. As a result, some links
might be brought up before the supervisor engine has configured forwarding for the links, causing traffic
to those links to be lost until the configuration is complete. This condition is especially disruptive if the
link is an MEC link and it is running in "ON" mode. This is why it is recommended that MEC ports
always have either PAgP or LACP mode of EtherChannel configured.
Note We recommend not configuring LACP independent mode (standalone-mode) for MEC because ports on
the VSS Standby switch (while it boots) come up tens of seconds before the control plane is fully
functional. This behavior causes a port to start working in independent mode and might cause traffic loss
until the port is bundled.
Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
5-12
Switches' Roles in a VSS
VSS Active
In-chassis Standby
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Linecard 2
Linecard N
Active chassis
Chapter 5 Configuring Virtual Switching Systems
VSL
VSS Standby
In-chassis Standby
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Linecard 2
Linecard N
Standby chassis